The use of ophthalmic lenses for the correction of ametropia is well known. For example, multifocal lenses, such as progressive addition lenses ("PAL's"), are used for the treatment of presbyopia. The surface of a PAL provides far, intermediate, and near vision in a gradual, continuous progression of vertically increasing dioptric power from far to near focus, or top to bottom of the lens.
PAL's are appealing to the wearer because PAL's are free of the visible ledges between the zones of differing dioptric power that are found in other multifocal lenses, such as bifocals and trifocals. However, an inherent disadvantage in PAL's is unwanted lens astigmatism, or astigmatism introduced or caused by one or more of the lens' surfaces. Generally, the unwanted lens astigmatism is located on either side of the near vision zone of the lens and, at or near its approximate center, reaches a maximum level that corresponds approximately to the near vision dioptric add power of the lens.
A substantial portion of the known progressive lens designs utilize one curved-surface equation to express the shape of the lens' progressive addition surface. This design method permits shifting of the areas of maximum, localized, unwanted astigmatism to the least used areas of the lens. Alternatively, it is known to design a progressive addition lens by dividing the progressive addition surface into a number of areas and using a curved-surface equation for each area to express the progressive addition surface's shape. Use of either of these design methods results in significant amounts of maximum, localized unwanted astigmatism. Thus, a need exists for a PAL design method and lens that further reduces maximum, localized unwanted astigmatism compared to known methods and lenses.